Field of the Invention
The present invention relates to a process for producing an oxalate diester by reacting an aliphatic alcohol, carbon monoxide and molecular oxygen in the presence of a novel catalyst. More particularly, the invention relates to a process for producing an oxalate diester by using a catalyst consisting essentially of metallic palladium or a palladium compound, a heteropoly-acid and a nitrogen compound.
An oxalate diester is hydrolyzed to form oxalic acid. An oxalate diester is used as a reagent for analysis and a solvent for perfume, and as intermediates for preparing organic compounds, such as ethylene glycol, oxamide, etc. Therefore, oxalate diesters are important compounds from industrial point of view.
In the prior art, a process for producing oxalate diesters comprising reacting an alcohol, carbon monoxide and molecular oxygen have been proposed in U.S. Pat. No. 3,393,136 dated July 16, 1968. The above known process comprises reacting an alcohol, carbon monoxide and molecular oxygen, under substantially anhydrous conditions, in the presence of a dehydrating agent, such as ortho ethyl formate and by using a catalyst comprising a platinum group metal, or a soluble salt or chelate compound thereof and a redox salt selected from the group consisting of soluble cupric and ferric salts. However, according to the process, a great amount of by-products are not only formed with an oxalate diester and selectivity for the oxalate diester is low, but also a dehydrating agent, such as ortho ethyl formate, has to be used. This is unpreferable from an industrial point of view.
Many catalysts for the reaction of an alcohol, carbon monoxide and molecular oxygen have been proposed. The catalysts are classified into the following two groups:
(1) (i) a platinum group metal, such as palladium, or a salt thereof, and (ii) (a) a copper or iron salt and a carbonate, a nitrate, a sulfate or a hydroxide of an alkali metal or an alkaline earth metal or (b) a copper or iron salt, a carbonate, a nitrate, a sulfate or a hydroxide of an alkali metal or an alkaline earth metal, and a tertiary amine, pyridine, urea or acetyl acetone; and
(2) palladium or its salt and nitric acid or an alkyl nitrite (refer to Japanese Patent Publication (Kokai) Nos. 15313/1978 and 41813/1979). However, in these processes, the oxalate diester is formed at a slow rate and at a low selectivity.
The catalyst comprising a palladium salt and a copper salt is also unsatisfactory; during the reaction, the copper salt undesirably reacts with the oxalate diester that has been produced in the reaction, and as a result, a copper oxalate precipitate is formed that causes a sudden drop in the activity of the catalyst. Furthermore, the copper oxalate precipitate is difficult to recover and regenerate for reuse. In the conventional technique, copper halide is primarily used as the copper salt, but the corrosive halogen component often attacks the reactor. U.S. Pat. No. 4,229,589 discloses a process for preparing an oxalic acid diester by bringing carbon monoxide into contact with a nitrous acid ester in liquid phase in the presence of metallic palladium or a salt thereof. This process solves the two problems, i.e. reduced catalytic activity and corrosion of the reactor, with the use of copper salt. But to achieve an industrially acceptable reaction rate, a large amount of the nitrous acid ester must be used, and this increases the chance of the formation of a by-product during the reaction and subsequent distillation step, which is certainly unsatisfactory from a commercial viewpoint.
We have screened many compounds in our search for a new catalyst that is free from the above described defects, and finally we found that when a heteropoly-acid and a certain nitrogen compound are used together with metallic palladium or a palladium compound as catalyst components, the intended reaction proceeds smoothly and the end compound, oxalate diester, is produced at high rate and with high selectivity.